1. Field of the Invention
This invention relates to cured epoxypolysiloxanes and their blends with epoxy-terminated silanes which are useful as release coatings for adhesive roll and sheet materials.
2. Description of the Prior Art
Coatings having specific release properties toward adhesives are widely used. Polydimethylsiloxanes, polymers containing predominantly dimethylsiloxane units, provide very low release coatings, e.g., 4 to 16 g/cm width, for products such as labels or large sheets which contain normally tacky and pressure-sensitive adhesives. These polymers are less useful as release coatings on the back surface of adhesive tape (back surface coating of adhesive tape is known as "low adhesion backsize" [LAB]) because their low release force can cause roll instability. LABs for tapes in roll form ideally exhibit release toward the adhesive of about 60 to 350 g/cm width. Polymers with higher release values make it increasingly difficult to use the tape and delamination of the adhesive from the substrate often can result. Coatings having release values less than 60 g/cm width are useful as components for release liners. Many non-silicone polymers, e.g., urethanes, find use as low adhesion backsizes for pressure-sensitive tapes because of their much higher release force than the polydimethylsiloxanes, typically greater than 200 g/cm width. Such non-silicone LAB coatings are exemplified in U.S. Pat. Nos. 2,532,011, 2,607,711, 2,876,894 and 3,342,625.
For products such as tapes and liners, coatings having specific release properties toward adhesives, which are intermediate between those of the polydimethylsiloxanes and conventionally used non-silicone LAB coatings, are highly desired. Many previous attempts to provide such coatings by modification of polydimethylsiloxanes or blending them with less effective release material, as disclosed in, for example, U.S. Pat. Nos. 3,328,482, 3,527,659, 3,770,687 and 3,891,745, have not met with total success because of many problems such as: (1) incompatability of components causing extensive migration of the silicone to the surface, (2) contamination of the adhesive with low molecular weight components giving adhesion loss, (3) nonreproducibility, e.g., inability to consistently achieve the desired release level, and (4) use of excessively high cure temperatures causing deterioration of the heat-sensitive substrate or tape backing.
One of the best products of the type described in the preceding paragraph is disclosed in U.S. Pat. No. 4,171,397, which relates to plural coatings presenting areas of (1) fluorochemical polymer and (2) cured silicone polymer.
Coating compositions exhibiting intermediate release properties towards adhesives, provided by reaction of an isocyanate with a hydroxyl or amine-containing organosiloxane, are described in U.S. Pat. No. 3,997,702.
Epoxypolysiloxanes have been previously used as polymer components to impart water repellency to textile and paper (U.S. Pat. Nos. 4,046,930 and 3,055,774), treating agents for filling synthetic fiber (U.S. Pat. No. 4,062,999) and adhesion additives (U.S. Pat. No. 4,033,924). It is believed that the use of epoxypolysiloxanes to provide effective release coatings with pressure-sensitive adhesives has not been previously described.